This invention relates to a chain tensioner for keeping constant the tension of mainly chains and toothed belts for driving camshafts.
A chain transmission device for driving camshafts includes a chain trained around a sprocket mounted to an end of a crankshaft and sprockets each mounted to one end of one of the camshafts. In order to keep constant the tension of the chain, a pivotable chain guide is pressed against the slack side of the chain, and an adjusting force is applied to the chain guide from a chain tensioner.
A conventional chain tensioner used for such a chain transmission device is disclosed in JP patent publication 2000-179632A. This chain tensioner includes a housing defining a cylinder chamber, and a plunger slidably inserted in the cylinder chamber to define a pressure chamber in the housing. A return spring is mounted in the pressure chamber to bias the plunger outwardly of the cylinder chamber. An oil supply passage is formed in the housing to communicate with the pressure chamber. At its end opening to the pressure chamber, a check valve is provided. The plunger is formed with a rod inserting hole that opens to the rear end surface of the plunger which faces the pressure chamber. A screw rod formed with an external thread having a serration-shaped section on its outer periphery is inserted in the rod inserting hole with the external thread in threaded engagement with an internal thread, which also has a serration-shaped section, formed on the inner surface of the rod inserting hole near its rear opening. With this arrangement, the pushing force applied to the plunger from the chain through the chain guide is damped by hydraulic oil sealed in the pressure chamber. Also, by the abutment of pressure flanks of the internal and external threads, rearward movement of the plunger is prevented.
While this pushing force is larger than the force of the return spring, the plunger is allowed to move backward with the screw rod rotating until the pushing force balances with the force of the return spring, thereby keeping constant the tension of the chain.
When the chain slackens, the plunger is pushed outwardly under the force of the return spring and simultaneously, the screw rod is moved backward while turning under the force of a spring mounted in the rod inserting hole, thereby re-tensioning the chain.
When mounting such a chain tensioner, the housing has to be fixed in position with the plunger pushed into the housing. It is therefore troublesome to mount such a chain tensioner.
A chain tensioner disclosed in JP patent publication 2005-42806A includes a housing defining a cylinder chamber and formed with a set groove in its inner periphery near the opening of the cylinder chamber. A set ring having a circumferentially cut-apart portion so as to be elastically and radially deformable is received in the set groove. The plunger has an engaging groove formed in the outer periphery thereof near its front end. When the plunger is pushed into the cylinder chamber, the set ring engages in the engaging groove, thereby keeping the plunger pushed in the cylinder chamber (initial set state). After the chain tensioner is mounted in position, the plunger is further pushed into the cylinder chamber to disengage the set ring from the engaging groove. By releasing the plunger thereafter, the plunger protrudes from the cylinder chamber under the force of the return spring.
The inventors of the present invention thought that by using the technique of retaining the plunger in the initial set state as disclosed in the latter patent publication in the chain tensioner disclosed in the former patent publication, it would be possible to retain the plunger in the initial set state, so that the chain tensioner can be more easily mounted in position.
However, if the technique disclosed in the latter patent publication is used in the chain tensioner disclosed in the former patent publication, if the plunger is turned in such a direction that the axial gap between the internal and external threads disappears due e.g. to vibration during transportation of the chain tensioner with the plunger retained in the initial set state, there is a possibility that it becomes impossible to axially push in the plunger, so that the set ring remains engaged in the engaging groove and thus the plunger is stuck in the initial set state.
An object of this invention is to provide a chain tensioner which can be easily mounted in position and which can reliably prevent the plunger from getting stuck in the initial set state.